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| Any moving charge will induce a magnetic field. This discovery
led physicists to believe that a magnetic field could induce an electrical
current in a conductor. In 1831 two people, Michael Faraday in the UK and
Joseph Henry in the US performed experiments that clearly demonstrated
that a changing magnetic field produces an induced EMF (voltage) that would
produce a current if the circuit was complete. The experimental set up was similar to the diagram. Two circuits were set up, one connected to a battery and a switch, the other connected only to a galvanometer, a current measuring device. When the switch was closed, a momentary deflection was noticed in the galvanometer after which the current returned to zero. When the switch was opened, the galvanonmeter deflected again momentarily, in the other direction. Current was not detected in the secondary circuit when the switch was left closed. |
| When the switch is closed, the current begins to flow and an induced magnetic field is set up around the primary coil. The current increases from zero to some value over a short period of time. The changing electrical current produced a changing magnetic field which is the cause of the induced current. When the switch is opened, the current decreases which results in a decreasing magnetic field. The result is an induced current in the secondary circuit, in the opposite direction. When a constant current flows in the primary circuit, the induced magnetic field is constant. A constant magnetic field does not induce a current in the secondary circuit. |
| The Direction of the Induced
EMF and Current There is an easy way to remember the direction of the induced EMF and current when a current moves through a magnetic field or vice versa. The 4th right hand rule can be used to determine the direction of the induced current. 1) Extend fingers in the direction of the external magnetic field. 2) Point the thumb in the direction of the velocity of the conductor, relative to the magnet. 3) The palm of your hand indicates the direction of the induced current. |
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Lenz's Law While the right hand rule is helpful for memorizing the direction of the induced current, it is important to understand why the current flows in the indicated direction. Let's consider a magnet moving towards a coil. We know that the coil moves relative to the conductor, so a current will be induced in the coil. What direction will the current flow?Let's guess that the current flows in the direction indicated by the diagram below. A small force acting on the magnet causes a changing magnetic field in the coil which will induce a current. If the current flowed in the indicated direction, an induced magnetic field would put the south pole opposite the north pole in the external magnet. |
| This would attract the magnet, causing it to accelerate.
The increased speed of the magnet would induce a larger current which would
pull even harder on the magnet and so on. If we think about this situation,
we see that a small amount of work input generates a larger output of energy.
This arrangement would violate the law of conservation of energy. If the induced current flows in the opposite direction, the induced current in the coil would set up a magnetic field with the north pole opposite to the external magnet. |
In order to generate a current, we would have to exert a force that would be opposed by the induced magnetic field. The harder we push on the magnet, the more repulsion we'd feel from the induced magnetic field. To increase the energy output, we would need to increase the work input. This would be consistent with what we know about the law of conservation of energy. Lenz's Law states that the induced current will always set up a magnetic field that will oppose the movement of the external magnetic field. |
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Determining the Magnitude of the Induced EMF The magnitude of the induced EMF can vary. The five factors that determine how large the induced EMF will be are: 1) The length of the conductor moving though the magnetic field. (l) 2) The number of wires (coils) moving through the magnetic field.(N) 3) the strength of the external magnetic field.(B) 4) the velocity of the conductor relative to the magnetic field.(v) 5) the angle between the velocity vector and the magnetic field vector.(q) Putting it all together: A more conceptual definition might include a picture of what is happening when we induce a current with a changing magnetic field. If there is a changing magnetic field around a conductor, the conductor must be moving relative to the field lines. |
| When the conductor "cuts" through the field lines, an EMF is induced. The more field lines the conductor cuts through, the greater the induced EMF. We can quantify the number of field lines by using the quantity magnetic flux, fB. In the accompanying diagram coil 2 has a greater induced EMF because the coil experiences a greater change in magnetic flux. |
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Generators: An Application of Electromagnetic Induction Since movement of a conductor through a magnetic field induces an EMF and current, we can illustrate how mechanical energy can be converted into electrical energy. Generators accomplish this task by turning a coil of wire that is placed in a magnetic field. A generator is essentially a motor that operates in reverse. |
| Looking at it from a different perspective, we can see how the current is induced and in what direction it flows. We can follow the generator through one complete turn below. |
| In the position 1, the conductors cut through the maximum
number of field lines, producing a maximum EMF and current.
In position 2, the conductors move parallel to the field lines, therefore no current is generated. |
In position 3 the conductors cut through the maximum number of field lines, producing a maximum EMF and current. The conductors move in the opposite direction as they did in position 1 and therefore the current moves in the opposite direction. In position 4, the conductors move parallel to the field lines, therefore no current is generated. As we can see, the current produced in generators changes directions twice per cycle. AC generators supply a circuit with alternating current. Slip rings make the connection between the turning coils and the circuit. |
| In DC generators,
a split ring commutator supplies pulsating DC to the circuit.
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Transmission of Electricity Over Long Distances Electrical current flowing through wire generates heat energy according to the equation H = I2Rt. Large currents result in much energy being lost as heat and unwanted inefficiency. We can minimize heat loss by transmitting electricity at larger voltages. We can step up voltage (and step down current) by using transformers. After the electricity has been transmitted over a distance, it can be stepped down to a more reasonable voltage using a step down transformer. |
The ratio of the coils in the secondary to the primary is equal to the ratio of the voltages in the secondary to the primary. N2 / N1 = V2 / V1 = I1 / I2 |
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